1. Field of the Invention
The invention relates to optical sensors for the paper and plastics flat sheet industries, and more particularly, to a spectroscopic sensor for measuring characteristics of flat sheet products.
2. Discussion of the Prior Art
In the paper and plastics industries, during the process of manufacture of flat sheet products, various sheet properties of multi-layered and single layer sheets can be detected with visible and infrared radiation while the sheet making machine is operating. Multiple components of the sheet including basis weight, coating weight, moisture content, opacity and layer thicknesses can be measured by sensors which detect the amount of radiation that the sheets absorb, transmit or reflect from a beam of infrared light or other radiation. In systems employing such sensors, radiation that interacts with the sheet is typically compared at two different wavelength bands, a reference wavelength band and a measurement wavelength band, to measure different properties of flat sheet products.
FIG. 1 illustrates a typical prior art sensor configuration. An infrared (IR) radiation source 103 directs a beam of IR radiation 101 towards a sample 110. The beam is transmitted through beam conditioning optics, such as collimating lenses and/or focusing lenses 190, 192, 146, 195. These lenses condition the optical radiation for optimal sensor efficiency. The optics 146, 195 in front of the detectors 145, 135, respectively, are typically focusing lenses and those adjacent to the sample are typically collimating or focusing lenses 190, 192. IR radiation is partly absorbed, reflected and transmitted by the sample 110 depending on its various properties. Beam splitter 120 splits the IR radiation into two separate beams 118 and 128. Each beam is directed to a separate bandpass filter 170 and 160, respectively, each of which is positioned and aligned immediately before detector 135 and 145, respectively. The bandpass filters 170 and 160 are configured to pass IR radiation at selected regions of the infrared spectrum. Any IR radiation not within the selected region of the spectrum is reflected by the filters back to the beam splitter 120.
Depending on the intensity of the radiation detected at the detector, the detector generates an analog electrical signal which may be converted to a digital signal for observation. The described sensor arrangement can measure different properties of the sample under observation. For instance, in the thickness measurement of thin plastic films, one of the two infrared bandpass filters only passes infrared radiation having wavelengths in a selected region of the infrared spectrum. This first region of the spectrum is called the “reference” region, and the associated detector is called the “reference” detector. The reference channel spectral range is located in a specific region of the IR spectrum which is not associated with a signature absorption band of the material or materials which the film is composed of. This reference channel however should be indicative of all other optical loss mechanisms in the sensor system and sheet that are not indicative of the optical absorption of the material being sensed. These other properties may include such things as scattering loss from the sheet or the insertion losses of the optical components used in the sensor system.
A second bandpass filter is associated with the second infrared detector and passes only wavelengths in a second selected region of the infrared spectrum. This second region of the spectrum is called the “measure” region. The detector associated with the “measure” region of the spectrum is called the “measure” detector. The wavelength region of the measure channel is chosen to encompass an IR spectral range that is characteristic of an optical absorption band associated with the material being sensed. The optical losses in the measure channel ideally include all the same losses that are associated with the reference channel in addition to the characteristic absorption band of the material being sensed. If a comparison is made between the optical signals detected by the reference and measure channels then we can ascertain the amount in terms of weight or thickness of the material being sensed.
The arrangement described consists of two single channel detectors—one reference and one measure. A pair of detectors will typically measure a single constituent component of the sheet such as total thickness, moisture or cellulose weight. However, often multiple characteristics or multiple components of the sample need to be measured. For example, when measuring a plastic sheet that is composed of multiple components, the relative concentration of each component must be determined. By necessity, this means looking at a wider spectrum so that multiple components of the sample can be measured at the same time. This can be done by stacking reference and measure channel pairs which have had their filters chosen for each constituent component of the sheet. Occasionally common reference channels can be used hence eliminating one or more of the reference channels.
An alternative to using multiple pairs of single detectors with filters is to use an optical spectrometer. An optical spectrometer can provide a convenient method of measuring properties of light over a larger, continuous portion of the spectrum while achieving improved spectral resolution. The spectrometer outputs light intensity as a function of wavelength over a specific range of wavelengths which is split up into pixels. For example the PSG2.2 InGaAs spectrometer from Zeiss covers a spectral range of 1000 to 2150 nm with 256 pixels and has a spectral resolution of 16 nm. Another example is the PSG1.7 InGaAs spectrometer from Zeiss which covers a spectral range of 960 to 1690 nm with 512 pixels and has a spectral resolution of 5 nm. This increased spectral resolution and convenience of a spectrometer is typically obtained at the expense of signal-to-noise ratio on the intensity measurement. Also, in the design of spectrometers one can trade off spectral resolution for spectral range.
Visible, near-IR and mid-IR sensors share a common need for large spectral range, high spectral resolution and high signal-to-noise ratio. Spectral range is needed for the sensor to address a large number of applications whereas spectral resolution and signal to noise ratio are key to good sensor accuracy and repeatability. However, these requirements are usually mutually exclusive. For example, a single detector plus filter combination that allows for high signal to noise ratio and good spectral resolution does not provide broad spectral range. Conversely, a compact spectrometer provides high spectral range with good spectral resolution but at the expense of signal-to-noise ratio and hence repeatability.
Additionally, spectrometers can cover a wide spectral range but, due to practical and technical reasons, a single spectrometer does not cover the entire range between the visible and the mid-IR range. Applications where this is an issue can be found in the plastic and paper industries. For example, a single sensor is desired to measure the thickness of thin plastic films on biaxial film production lines. The very thin films can be measured using interferometry in the visible or near-infrared spectrum where absorption is weak, whereas thicker films (greater than 15-20 μm) and edge beads are measured using absorption further out in the near-infrared spectrum. Commonly assigned U.S. Pat. No. 7,088,456 to Germanenko et al. discloses a system and method for analyzing characteristics of thin films using IR sensors; this patent is incorporated herein by reference. Similarly, moisture or coat weight in paper applications and thickness in plastic applications are measured in the near-IR spectrum while opacity is measured in the visible spectrum. In the above described cases, a single spectrometer or single channel detector and filter combinations cannot fully address the measurement needs. Many types of spectrometers exist, including array spectrometers, Fourier Transform Infrared (FTIR) spectrometers, Acousto-Optic Tunable Filter (AOTF) spectrometers, Linear Variable Filter (LVF) spectrometers and Fabry-Perot spectrometers. However, at present, no single spectrometer is available with required noise characteristics, spectral resolution (i.e. number of elements), and wavelength range.
In prior art optical sensors are designed either with a spectrometer or individual filter and detector combinations.